/*
 * Copyright (C) 2006 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import static com.google.common.collect.Lists.newArrayListWithCapacity;
import static java.util.Collections.unmodifiableList;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.ImmutableCollection;
import com.google.common.collect.Lists;

import java.util.Collections;
import java.util.List;
import javax.annotation.CheckForNull;

import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Aggregate future that collects (stores) results of each future.
 */
@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
abstract class CollectionFuture<V extends @Nullable Object, C extends @Nullable Object>
        extends AggregateFuture<V, C>
{
    /*
     * We access this field racily but safely. For discussion of a similar situation, see the comments
     * on the fields of TimeoutFuture. This field is slightly different than the fields discussed
     * there: cancel() never reads this field, only writes to it. That makes the race here completely
     * harmless, rather than just 99.99% harmless.
     */
    @CheckForNull
    private List<@Nullable Present<V>> values;

    CollectionFuture(
            ImmutableCollection<? extends ListenableFuture<? extends V>> futures,
            boolean allMustSucceed)
    {
        super(futures, allMustSucceed, true);

        List<@Nullable Present<V>> values =
                futures.isEmpty()
                        ? Collections.<@Nullable Present<V>>emptyList()
                        : Lists.<@Nullable Present<V>>newArrayListWithCapacity(futures.size());

        // Populate the results list with null initially.
        for (int i = 0; i < futures.size(); ++i)
        {
            values.add(null);
        }

        this.values = values;
    }

    @Override
    final void collectOneValue(int index, @ParametricNullness V returnValue)
    {
        List<@Nullable Present<V>> localValues = values;
        if (localValues != null)
        {
            localValues.set(index, new Present<>(returnValue));
        }
    }

    @Override
    final void handleAllCompleted()
    {
        List<@Nullable Present<V>> localValues = values;
        if (localValues != null)
        {
            set(combine(localValues));
        }
    }

    @Override
    void releaseResources(ReleaseResourcesReason reason)
    {
        super.releaseResources(reason);
        this.values = null;
    }

    abstract C combine(List<@Nullable Present<V>> values);

    /**
     * Used for {@link Futures#allAsList} and {@link Futures#successfulAsList}.
     */
    static final class ListFuture<V extends @Nullable Object>
            extends CollectionFuture<V, List<@Nullable V>>
    {
        ListFuture(
                ImmutableCollection<? extends ListenableFuture<? extends V>> futures,
                boolean allMustSucceed)
        {
            super(futures, allMustSucceed);
            init();
        }

        @Override
        public List<@Nullable V> combine(List<@Nullable Present<V>> values)
        {
            List<@Nullable V> result = newArrayListWithCapacity(values.size());
            for (Present<V> element : values)
            {
                result.add(element != null ? element.value : null);
            }
            return unmodifiableList(result);
        }
    }

    /**
     * The result of a successful {@code Future}.
     */
    private static final class Present<V extends @Nullable Object>
    {
        V value;

        Present(V value)
        {
            this.value = value;
        }
    }
}
